In a DS—CDMA system (Direct Sequence Code Division Multiple Access), the system carries on communication with different code or code words allocated to each subscriber when different subscribers work simultaneously on the same frequency. A communication mode makes capacity and performance of the system depend on interference on a subscriber to a great extent. In a CDMA communication system, any method of reducing interference on a subscriber will improve system performance. Power control technique is an effective means of reducing interference between channels in the system and is a key technique in a CDMA communication system.
There are two types of power control methods: continuous power control and fixed step length power control. Theoretically, the continuous power control method is superior to the fixed step length control method in performance. However, in view of complexity for realization, the later is still an effective method in implementation of power control in the system. The present invention relates to a method for fixed step length power control.
In a power control of forward links, a resource allocation to subscribers is really a distribution of total transmitting power among different subscribers by a base station. The principle on power control is to distribute minimum necessary power to each subscriber according to a transmitting power control instruction (hereinafter referred to as “TPC instruction”), i.e. each subscriber received after detection of received signal quality under the guarantee of communication quality of each subscriber.
With increasing of numbers of access subscribers in a cell, the transmitting power distributed by a base station increases too, and transmitting power finally reaches maximum at a base station. Then, the forward power control enters a saturation state where a base station has no way to get more additional transmitting power to meet the requirement of TPC instructions from the subscribers in a cell, whereby the forward power control needs a saturation protection to balance transmitting power for all subscribers under the condition that the fundamental principle on power control be satisfied.
There are two types of saturation states in forward power control:
1) Because of the influence of shadow fading and Rayleigh fading in communication channels, the balance of power control is only a dynamic balance. The subscriber's balance parameter (power or SIR) still keeps fluctuation in a stable state. When an approximately full load occurs in a cell, power requirement caused by fluctuation of balance parameter may cause the total transmitting power required by a subscriber to be beyond total transmitting power from a base station.
2) When a base station is approximately fully loaded, the receiving quality acutely deteriorates if any subscriber in a cell enters a deep fading. As a result, the subscriber requires a base station to distribute more power. Then, the base station enters a saturation state.
Two methods have been adopted under the above mentioned two conditions. The first method is to use a hand-over, i.e., a part of subscribers is handed over to adjacent cells. In fact, it is a move strategy of mobile stations. Although the method will not influence other subscribers in a system, hand-over cannot be done anywhere because of the actual location of a mobile station. For example, a subscriber in a deep-fading condition is very far from the other cells, or all adjacent cells are fully loaded and are not able to supply any channel for hand-over.
The second method is to use a saturation protection. The limited power is distributed to a subscriber who needs the most. It is guaranteed that communication quality for all subscribers are acceptable without significantly reducing the communication performance of the other subscribers. Although the saturation protection method may still cause subscribers to reduce communication quality, it is more feasible than the hand-over move strategy.
At present, there is little research done on the subject of saturation protection for forward power control. U.S. Pat. No. 5,794,129 by NEC and U.S. Pat. No. 5,689,815 by OKI have discussed a couple of methods on the subject as follows:
1) Average reduction of transmitting power of all subscribers. In this method, when a system enters a saturation state, transmitting power of all subscribers is reduced by a fixed value D1 to see whether it meets the requirement. If it is met, saturation state leaves. Otherwise, transmitting power for all subscribers is reduced again by D1 to see whether it meets the requirement. The action repeats, if necessary, till the requirement is met.
2) Reduce transmitting power of a subscriber who uses high power. In the method, it is needed to determine a transmitting power threshold. A subscriber who uses power more than the threshold is defined as high power subscriber. As saturation occurs, the transmitting power of a high power subscriber is reduced by a fixed value D2 to see whether it can meet the requirement. If it is met, saturation state leaves. Otherwise, transmitting power of a high power subscriber is reduced again by value D2. The action repeats till the requirement is met.
3) Reduce the next transmitting power in proportional to existing transmitting power. In the method, if saturation state occurs, transmitting power of all subscribers is reduced by a fixed proportional value C to see whether it can meet the requirement. If it can be met, saturation state leaves. Otherwise, transmitting power of all subscribers is reduced again by a fixed proportional value C. The action repeats till the requirement is met.
From the above, the first method is not proved well by simulation (see simulation demonstration later). In the second method, selecting a proper threshold value of a high power subscriber is very important. An improper threshold value selected may reduce system performance. Accordingly, its realization is very difficult. The third method cannot meet the requirement of adjustment of fixed step length power.
Accordingly, there is a need for an improved protection method and a power control apparatus to meet the requirement of adjustment of fixed step length power, to balance transmitting power for all subscribers under the condition of a fundamental principle that power control be satisfied, and to make a prompt response to a subscriber who needs power increasing urgently.